Systems and methods for controlling rollback in continuously variable transmissions

ABSTRACT

A continuously variable transmission capable of operating in a forward direction or reverse direction may be controlled in the reverse direction by providing an initial skew angle in a first skew direction, followed by a set or sequence of skew angle adjustments in an opposite direction to prevent runaway or other unintended consequences. A continuously variable transmission may include a timing plate to maintain all planets at an angle or within a range of an angle in forward and reverse operations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/102,437, filed Aug. 13, 2018, entitled “Systems And Methods ForControlling Rollback In Continuously Variable Transmissions,” which is acontinuation of U.S. patent application Ser. No. 14/996,743, filed Jan.15, 2016, entitled “Systems And Methods For Controlling Rollback InContinuously Variable Transmissions,” now U.S. Pat. No. 10,047,861,which are hereby incorporated by reference herein. This application isalso related to U.S. Pat. Nos. 8,313,404, 8,469,856, and 8,888,643, allof which are incorporated by reference herein.

FIELD OF THE DISCLOSURE

Embodiments disclosed herein may be directed to continuously variabletransmissions. In particular, embodiments may be directed toball-planetary type continuously variable transmissions intended forforward rotation.

BACKGROUND

The term “continuously variable planetary transmission” (or “CVP”) mayrefer to a variable ratio planetary drive assembly capable oftransmitting continuous and stepless power (speed and torque). A CVP maybe a continuously variable subassembly (or “variator”) of a continuouslyvariable transmission or, where there are no additional elements thatmodify speed and torque, the CVP may be referred to as a continuouslyvariable transmission (“CVT”). Ball-planetary continuously variabletransmissions (CVTs) generally utilize a plurality of spherical rollingelements (also commonly referred to as balls, planets, or spheres)interposed between an input ring and an output ring, and in contact witha sun. A CVP capable of operating in either a forward direction or areverse direction may be referred to as an infinitely variabletransmission (“IVT”).

SUMMARY

Embodiments disclosed herein may overcome the shortcomings of the priorart with systems and methods for controlling rollback in a continuouslyvariable transmission.

In one broad respect, embodiments may be directed to a system or methodfor controlling rollback in a continuously variable transmission. Whenoperating in a forward (design) direction, feedback is generallyprovided by the geometry of carrier slots. During reversed operation(rotation opposite to design), feedback is provided by skew actuatorcommands generated by algorithms in a control module. The control modulemay be integrated with an actuator or comprise a set of instructionsstored in memory on a controller communicatively coupled to an actuator.

In another broad respect, embodiments may be directed to a system ormethod for controlling rollback in a continuously variable transmissionoperating in a reverse direction. In some embodiments, at the onset ofreverse operation, a set of commands causes an actuator to change skewangle .zeta. (zeta) of a plurality of planet axles in a direction thatwill result in a change in tilt angle .gamma. (gamma) towards reductionduring rotation opposite to design. The number and timing of the set ofcommands overcomes inaccuracies in skew angle .zeta. (zeta) due tomachining error or overcomes droop in skew angle .zeta. (zeta) due toload. The set of commands may include a first command to cause at leastone planet carrier to rotate in a first direction. In some embodiments,the set of commands may include a second command to hold at least oneplanet carrier at a fixed skew angle. In some embodiments, the set ofcommands may include one or more commands executed after a first orsecond command as needed to rotate at least one planet carrier in asecond rotation direction opposite the first rotation direction. Themethod may be performed by executing a set of instructions stored in amemory integral to the actuator or comprise a set of instructions storedin memory on a controller communicatively coupled to the actuator.

In another broad respect, embodiments may be directed to a system ormethod for controlling rollback in a continuously variable transmissioncapable of operating in a forward or reverse direction. At theexpectation of reverse operation, a set of commands causes an actuatorto offset skew angle .zeta. (zeta) for a plurality of planet axles in adirection to result in a change in the tilt angle .gamma. (gamma)towards reduction for reverse operation. The set of commands, includingthe initial skew angle and any subset angles, is determined based on oneor more of a geometry of the CVT, a desired operating speed or torque ofthe CVT and a determined input torque or speed of the CVT. The set ofcommands overcomes inaccuracies in skew angle .zeta. (zeta) due tomachining error and overcomes droop in skew angle .zeta. (zeta) due toload. The method may be performed by executing a set of instructionsstored in a memory integral to the actuator or comprise a set ofinstructions stored in memory on a controller communicatively coupled tothe actuator.

In another broad respect, embodiments may be directed to a system ormethod for managing rollback in a continuously variable transmission.Whenever the transmission is stopped, and the next action isindeterminate, a command is sent to an actuator for offsetting skewangle .zeta. (zeta) for a plurality of planet axles in a direction thatwill result in a change in the tilt angle .gamma. (gamma) towardsreduction if rotation direction is reverse. The command is sufficient toovercome inaccuracies in skew angle .zeta. (zeta) due to machining errorand overcomes droop in skew angle .zeta. (zeta) due to load. The methodmay be performed by executing a set of instructions stored in a memoryintegral to the actuator or comprise a set of instructions stored inmemory on a controller communicatively coupled to the actuator.

In another broad respect, embodiments may be directed to a system ormethod for controlling rollback in a continuously variable transmission.During rotation opposite to design, skew angle .zeta. (zeta) may becontinuously monitored as the change in tilt angle .gamma. (gamma) forthe drive approaches reduction. If skew angle .zeta. (zeta) isdetermined to be increasing due to positive feedback (e.g., angled guideslots increasing skew angle .zeta. (zeta)), rotation of at least onecarrier in an opposite direction may be used to counteract the positivefeedback. In some embodiments, if during rotation opposite to design theskew angle .zeta. (zeta) is offset in a direction that causes a changein the tilt angle .gamma. (gamma) towards reduction, then as the tiltangle .gamma. (gamma) changes towards reduction, the angled guide slotswill cause an increase in the skew angle .zeta. (zeta). In someembodiments, to prevent runaway adjustments, a subsequent change in theskew angle .zeta. (zeta) back towards zero skew angle follows the ratiochange. A method may be performed by executing a set of instructionsstored in a memory integral to an actuator or comprise a set ofinstructions stored in memory on a controller communicatively coupled toan actuator.

In another broad respect, embodiments may be directed to a method formanaging the skew angle in a continuously variable transmission. Atargeted continuous operating condition for rotation opposite designcomprises a rotation position where the planet axle ends nearest aninput end of the CVP contact the centermost limit of the input carrier'sguide slot. In some embodiments, a continuous skew angle .zeta. (zeta)may be limited to the minimum skew angle .zeta. (zeta) required tomaintain a reduction rotation effort for each of the planets in an arrayof planets. In some embodiments, the continuous skew angle .zeta. (zeta)may be limited with consideration to machining errors or an unexpectedchange in external load and ratio droop.

In another broad respect, embodiments may be directed to a system forcontrolling skew angle in a continuously variable transmission. Aslotted timing plate may be used to limit the error in ratio angle anysingle planet may have in relation to the mean ratio of the remainingplanets. The timing plate may be a free turning disc with radial guideslots placed axially between the carrier halves. Each of the planetaxles extends through the timing plate and engages the carrier guideslots at one end of the drive. Tolerances of timing plate slots allowthe carrier guide slots to be the primary circumferential alignmentfeature for the planets. The angle or tolerances of slots in a timingplate may be based on slots formed in at least one carrier.

In another broad respect, embodiments may be directed to a system forcontrolling skew angle in a continuously variable transmission. Aslotted timing plate having slots with tolerances and oriented at anangle other than perpendicular to an axis of rotation may be used tolimit the error in ratio angle any single planet may have in relation tothe mean ratio of the remaining planets. The timing plate may be a freeturning disc with radial guide slots, and may be positioned axiallyoutside of the carrier halves. Each of the planet axles extends throughthe carrier guide slots and engages the timing plate at one end of thedrive (i.e., at an input or an output of the CVT). Tolerances of timingplate slots allow the carrier guide slots to be the primarycircumferential alignment feature for the planets.

In another broad respect, embodiments disclosed herein may be directedto a variator having a sun, a plurality of planets, and first and secondrings. The plurality of planets may be interposed between the first andsecond rings, and further in contact with and rotatable about the sun.An offset radial slot timing plate may enhance the control by ensuringeach planet in the plurality of planets is within a controlled ratioangle of the whole and within a limited skew angle .zeta. (zeta) of thewhole. The timing plate may be a free turning disc with offset radialguide slots placed axially between the array of planets and one of thecarriers. Each of the planet axles extends through a timing plate slotand engages a carrier guide slot. The timing plate slots have toleranceslarge enough to allow the carrier guide slots to be the primarycircumferential alignment feature for the planets. The angle between thetiming plate slots and the carrier guide slots is non zero.

In another broad respect, embodiments disclosed herein may be directedto a variator having a sun, a plurality of planets, and first and secondrings. The plurality of planets may be interposed between the first andsecond rings, and further in contact with and rotatable about the sun.An offset radial slot timing plate may enhance the control by ensuringeach planet in the plurality of planets is within a controlled ratioangle of the whole and within a limited skew angle .zeta. (zeta) of thewhole. The timing plate may be a free turning disc with offset radialguide slots placed axially outside the array of planets and axiallyoutside one of the carriers. Each of the planet axles extends through acarrier guide slot and engages a timing plate slot. The carrier guideslots have tolerances large enough to allow the timing plate slots to bethe primary circumferential alignment feature for the planets. The anglebetween the timing plate slots and the carrier guide slots is non zero.

In another broad respect, embodiments disclosed herein may be directedto a variator having a sun, a plurality of planets, first and secondrings, first and second carriers, and a timing plate used to limiterrors in ratio that any single spherical planet may have in relation tothe mean ratio of the plurality of planets. The timing plate may begrounded relative to the carrier located opposite the plurality ofplanets. The timing plate may be grounded due to a direct couplingbetween the timing plate and the carrier or may be grounded to anelement that is also grounded relative to the carrier.

In another broad respect, embodiments disclosed herein may be directedto a variator having a sun, a plurality of planets, first and secondrings, first and second carriers, and a timing plate used to limit theerror in ratio that any single planet may have in relation to the meanratio of the remaining planets. The timing plate may be counter-timed tothe carrier located on the same side of the plurality of planets. Inother words, if the timing plate is located near an input carrier on afirst side of the plurality of planets, the timing plate may becounter-timed relative to the input carrier. Counter-timing the timingplate with the first carrier may be accomplished by a gear mechanism.

These, and other, aspects will be better appreciated and understood whenconsidered in conjunction with the following description and theaccompanying drawings. The following description, while indicatingvarious embodiments of the invention and numerous specific detailsthereof, is given by way of illustration and not of limitation. Manysubstitutions, modifications, additions or rearrangements may be madewithin the scope of the disclosure, and the disclosure includes all suchsubstitutions, modifications, additions or rearrangements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, and 1D depict simplified views of a CVT, illustratingone embodiment of a control mechanism for a ball-planetary typeinfinitely variable transmission

FIG. 2A depicts a graph of tilt angle and skew angle over time,illustrating operation of one embodiment of a continuously variabletransmission in a design direction;

FIG. 2B depicts a graph of tilt angle and skew angle over time,illustrating operation of one embodiment of a continuously variabletransmission in a reverse direction;

FIG. 3A depicts a graph of tilt angle and skew angle over time,illustrating one method of managing rollback in a continuously variabletransmission according to one embodiment;

FIG. 3B depicts a graph of tilt angle and skew angle over time,illustrating one method of managing rollback in a continuously variabletransmission according to one embodiment;

FIG. 4 depicts a flow chart illustrating one method for controllingrollback in a continuously variable transmission according to oneembodiment;

FIG. 5A depicts a partial view of one embodiment of a system utilizing atiming plate for controlling a continuously variable transmission duringreverse operation;

FIG. 5B depicts an exploded view of one embodiment of a system utilizinga timing plate for use in controlling a continuously variabletransmission during reverse operation; and

FIGS. 5C and 5D depict views of one embodiment of a system includingcarrier plates with control via a floating timing plate interposedbetween the carrier plates.

DETAILED DESCRIPTION

Various features and advantageous details are explained more fully withreference to the non-limiting embodiments that are illustrated in theaccompanying drawings and detailed in the following description.Descriptions of well-known starting materials, processing techniques,components and equipment are omitted so as not to unnecessarily obscurethe features and advantages they provide. It should be understood,however, that the detailed description and the specific examples, whileindicating preferred embodiments, are given by way of illustration onlyand not by way of limitation. Various substitutions, modifications,additions and/or rearrangements within the spirit and/or scope of theunderlying concepts will become apparent to those skilled in the artfrom this disclosure.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,product, article, or apparatus that comprises a list of elements is notnecessarily limited only those elements but may include other elementsnot expressly listed or inherent to such process, product, article, orapparatus. Further, unless expressly stated to the contrary, the use ofthe term “or” refers to an inclusive “or” and not to an exclusive (orlogical) “or.” For example, a condition “A or B” is satisfied by any oneof the following: A is true (or present) and B is false (or notpresent), A is false (or not present) and B is true (or present), orboth A and B are true (or present).

Additionally, any examples or illustrations given herein are not to beregarded in any way as restrictions on, limits to, or expressdefinitions of, any term or terms with which they are utilized. Instead,these examples or illustrations are to be regarded as being describedwith respect to one particular embodiment and as illustrative only.Those of ordinary skill in the art will appreciate that any term orterms with which these examples or illustrations are utilized willencompass other embodiments which may or may not be given therewith orelsewhere in the specification and all such embodiments are intended tobe included within the scope of that term or terms. Language designatingsuch non-limiting examples and illustrations includes, but is notlimited to: “for example,” “for instance,” “e.g.,” “in one embodiment.”

Embodiments of the invention disclosed here are related to the controlof a variator and/or a CVT using generally spherical planets each havinga tiltable axis of rotation that can be adjusted to achieve a desiredratio of input speed to output speed during operation. In someembodiments, adjustment of said axis of rotation involves angulardisplacement of the planet axis in a first plane in order to achieve anangular adjustment of the planet axis in a second plane, wherein thesecond plane is substantially perpendicular to the first plane. Theangular displacement in the first plane is referred to here as “skew,”“skew angle,” and/or “skew condition.” For discussion purposes, thefirst plane is generally parallel to a longitudinal axis of the variatorand/or the CVT. The second plane can be generally perpendicular to thelongitudinal axis. In one embodiment, a control system coordinates theuse of a skew angle to generate forces between certain contactingcomponents in the variator that will tilt the planet axis of rotationsubstantially in the second plane. The tilting of the planet axis ofrotation adjusts the speed ratio of the variator. The aforementionedskew angle, or skew condition, can be applied in a plane substantiallyperpendicular to the plane of the page of FIG. 1, for example.Embodiments of transmissions employing certain inventive skew controlsystems for attaining a desired speed ratio of a variator will bediscussed.

The following description may be easier to understand in a specificcontext, particularly when considering a variator configured such thatpower is input via a first ring and exits via a second ring, with powernot passing through a sun. In the context of the configurationillustrated in FIGS. 1A, 1B, and 1C, planet carrier 114 or 124 may benon-rotating with respect to a main axle such as main axle 101, theinput is via a first ring 112A and the output is via a second ring 112B(also referred to as a “ring to ring” configuration). The actions andeffects are the same for any configuration, such as input to a pair ofcarriers or to the sun, as long as the description is normalized torelative rotation of the rings to the carrier.

FIGS. 1A, 1B and 1C depict simplified views of an exemplary CVPcomprising a plurality of planets for continuously transferring power.In particular, FIG. 1A depicts CVP 100 having a plurality of planets 110distributed about main axle 101 defining longitudinal axis 105. Mainaxle 101 is a rigid member for supporting other elements and fortransmitting power from a power source to the elements or from theelements to a power load. Main axle 101 may be solid or may have atleast a portion with a hollow passage for allowing fluid flow or toaccommodate sensors, wires, control mechanisms, rods, shafts, otheraxles, conduits, reservoirs, etc. Longitudinal axis 105 is an axis ofrotation for planets 110, traction rings 112, sun 102 and planetcarriers 114 and 124, and may also be referred to as a main axis or acenter axis.

In addition to elements rotating about main axis 105, each planet 110has a tiltable planet axle 111. Each planet axle 111 extends through aplanet 110 and defines axis of rotation 115 for that planet 110. Similarto main axle 101, planet axles 111 are rigid members. Planet axles mayalso be solid or hollow for enabling fluid flow through planets 110.

Power may be input to planets 110 via ring 112A or 112B or to sun 102via main axle 101, and transferred via planets 110 to sun 102 or ring112A or 112B as needed. As described above, planets 110 are rotatableabout tiltable planet axles 111. Control of how power is transferredacross planets 110 is based on a tilt angle .gamma. (gamma). Tilt angle.gamma. (gamma) is used herein to refer to a projection of an anglebetween planet axis of rotation 115 and longitudinal axis 105 in theplane containing both axes, and may also be referred to as “ratioangle.”

Embodiments disclosed herein may take advantage of the architecture ofCVTs that allows skew shifting (i.e., imparting a skew angle to cause astepless change in tilt angle .gamma.). FIGS. 1B-1D depict side and topviews of one embodiment of a CVT, illustrating slot angles and angulardisplacement and their effects on skew angle. As depicted in FIGS. 1A,1B, 1C, and 1D, carrier 114 has slots 116 configured to retain ends 111Aof planet axles 111 such that ends 111A of planet axles 111 may movealong slots 116 (independently of axle ends 111B retained in slots 126of carrier 124), enabling a change in skew angle .zeta. (zeta), causinga change in tilt angle .gamma. (gamma) to provide continuous andstepless adjustment of the speed ratio of variator 100. As depicted inFIG. 1B, slots 116 (or 126) may be oriented at a bias angle B of zerodegrees, where bias angle B is relative to a construction line L.sub.cof carrier 114 (or 124) extending radially outward perpendicular to axisof rotation 115 (extending out of the page).

As depicted in FIGS. 1A, 1B and 1C, carrier 124 has slots 126 configuredto retain ends 111B of planet axles 111 such that ends 111B of planetaxles 111 may move along slots 126 (independent of axle ends 112A),enabling a change in skew angle .zeta. to cause a change in tilt angle.gamma. (gamma) to provide a continuous (stepless) change in the speedratio of variator 100.

In some embodiments, carrier 114 is rotatable relative to carrier 124.In other embodiments, carrier 124 is rotatable relative to carrier 114.The angle of relative rotation between carriers 114 and 124 may beadjusted based on a desired skew angle .zeta. (zeta), a target tiltangle .gamma. (gamma), or a desired speed ratio (SR). In other words, ifslots 116 in carrier 114 have a different angle or orientation relativeto slots 126 in carrier 124, then when carriers 114 and 124 rotaterelative to each other, the ends 111A, 111B of planet axles 111 maytranslate or rotate within slots 116 or 126, causing a skew angle(.zeta.) to be applied to planets 110 to cause a change in tilt angle.gamma. (gamma), causing a change in speed ratio (SR). Ends 111A, 111Bof planet axles 111 may be configured to allow for linear motion as wellas rotational motion of planet axles 111.

When planet axles 111 are oriented such that axes of rotation 115 areparallel with center axis 105 (i.e., tilt angle .gamma. (gamma)=0), therotational speed and torque transmitted across planets 110 to ring 112Bis substantially equivalent to the rotational speed and torque appliedto ring 112A (minus losses due to friction, tolerances and the like).When power is transmitted from ring to ring (e.g., from ring 112A toring 112B or from ring 112B to ring 112A) and planet axles 111 aretilted at a non-zero tilt angle (i.e., tilt angle .gamma. (gamma) isgreater than or less than 0), the CVP is considered to be operating ineither underdrive or overdrive, and the rotational speed and torque areat some other ratio. The term “underdrive” is used herein to refer to atransmission ratio that causes in an increase of torque from the inputto the output of a transmission. Underdrive may also refer to a decreasein rotational speed from the input to the output of a transmission, andmay also be referred to as “reduction.” When planet axles 111 are at apositive tilt angle .gamma. (gamma) greater than 0 such that axes ofrotation 115 are not parallel with center axis 105, ring 112Bexperiences an increase in torque and a decrease in rotational speed.The term “overdrive” is used herein to refer to a transmission ratiothat causes a decrease of torque from the input to the output of atransmission. Overdrive may also refer to an increase in rotationalspeed from the input to the output of a transmission, and may also bereferred to as “speed up.” When planet axles 111 are at a negative tiltangle .gamma. (gamma) greater than 0, CVP 100 is considered to be inoverdrive and ring 112B experiences a decrease in torque and an increasein rotational speed. The principles apply whether the power path is froma ring to ring, ring to sun, or sun to ring in that the relationshipbetween the skew forces and the skew direction are constant.

The value of the tilt angle .gamma. (gamma) (including positive ornegative) may be controlled through the use of carriers 114, 124.Carriers 114, 124 are structures that control the relative rotationangle .PSI. (psi) between ends 111A, 111B of planet axles 111. Carriers114, 124 control the absolute rotational angle between the planetpositions and an inertial reference frame. A change in the relativerotational angle between first and second carriers 114, 124 may bereferred to as .PSI. (psi) or “carrier shift.” It should be noted thatcarrier rotation refers to something other than, for example, “ratioshift” or “gamma shift.” Furthermore, the term “rotation angle” is usedherein to refer to a relative rotational angle between carriers 114 and124. For ease of understanding, throughout this document carrier 114 isreferred to as being placed at the input of the variator, and carrier124 is referred to as located at the output of the variator. Carriers114 and 124 have slots 116, 126 configured to retain ends 111A, 111B ofplanet axles 111 such that ends 111A, 111B may translate along slots116, 126 and may further rotate or experience other motion.

Slots 116, 126 each have a length L, a width W, and a slot angle .THETA.(theta). The lengths of slots 116, 126 extend inside a pitch diameter(D.sub.P) of carrier 114, 124. The widths of slots 116, 126 allow ends111A or 111B of planet axles 111 to translate or rotate. However, if thewidth of any slot 116, 126 is outside a tolerance for all slots 116, itis possible for one planet 110 to behave different than other planets110 and control of a CVP becomes more difficult. One effect of a CVPbeing more difficult to control is a decreased efficiency of the CVP.Slot angle .THETA. (theta) is defined at the projected intersection (P)of the centerline of the skew guide feature (i.e., slot 116 or 126), aradial line L.sub.R normal to center axis 105 and pitch diameter D.sub.Pof the array of traction planets 110, wherein the projection plane isnormal to center axis 105. Slot angle .THETA. may also be referred to asan offset radial angle, a skew slot angle, or a guide slot angle. Theterm “radial” generally describes a line, groove or slot normal tocenter axis 105. A second description of an offset radial feature is afeature tangent to a circle of non-zero radius concentric to the centeraxis.

Ratio rotation may be controlled by applying a skew angle .zeta. (zeta)to planet axles 111. Skew refers to an angle from a plane containingplanet axis 115 to a plane containing center axis 105. A skew angle.zeta. (zeta) may refer to an included angle between the projection of askew guide feature and a radial line L.sub.R normal to center axis 105,wherein the projection plane is normal to center axis 105.

During normal direction operation of CVT 100, the geometry of carriers114, 124 adds negative feedback when tilt angle .gamma. (gamma) changes,which contributes to system stability during ratio change. One suchnegative feedback geometry involves carriers 114 with at least one setof carrier guide slots 116 or 126 that are not purely radial but insteadare angled with respect to a radial plane. The angle .beta. (beta) ofslots 116 may be singular (i.e., constant) or may be different at eachradial increment. Slots 116 or 126 may be straight or curved.Advantageously, angled slots 116 or 126 utilize a positive change ofratio angle to cancel a positive skew angle .zeta. When a CVT isrotating in the normal direction (also referred to as the design orrolling direction), any bias of the CVT is countered, the CVT remains atthe desired skew angle .zeta. (zeta) until a tilt angle .gamma. (gamma)is achieved, and the CVT is stable. Thus, only an initial skew angleinput is necessary to achieve a target tilt angle output.

An undesirable effect can occur when a ball type variator operates in areverse direction. Namely, if a CVT is configured to induce a tilt angleby applying a skew angle .zeta. (zeta) in the rolling direction andrelies on negative feedback to reduce skew angle .zeta. (zeta) as thetilt angle .gamma. (gamma) changes, then if the direction of operationis reversed and the negative feedback becomes positive feedback (i.e.,the feedback becomes positive such that a skew angle .zeta. (zeta)inducing a tilt angle change is positively reinforced as the tilt angle.gamma. (gamma) changes) the CVT may become unstable, and might continueto change tilt angle .gamma. (gamma) to an extreme underdrive oroverdrive condition.

FIGS. 2A and 2B depict diagrams illustrating operation of a CVTaccording to one embodiment. As a general note, solid lines representactive control or input into the system, and dashed lines representeffects. Thus, a change in an actuator position (e.g., an externalcommand) intended for causing a desired rotation of a carrier 114 or 124will be represented by a solid line, whereas if the same carrier 114 or124 is rotated due to the geometry of the CVT, that movement isrepresented by a dashed line.

FIG. 2A depicts a diagram, illustrating changes in tilt angle .gamma.(gamma) 220 and skew angle 210 over time for planet axles 111 (andtherefore planet axes of rotation 115) in a CVT, illustrating arelationship between skew angle and tilt angle .gamma. (gamma) for a CVToperating in a design operation. As depicted in FIG. 2A, skew angle 210is controlled by a CVT during a first time period 210 a until the skewangle reaches a desired skew angle 210 b. In response, tilt angle 220“follows” the skew angle over time 220 a to a target tilt angle 220 b,and skew angle 210 returns to zero over time 210 c. In other words,during operation in the design direction, a first rotation of carrier114 or 124 relative to carrier 124 or 114 to target skew angle 210 binduces planet axle ends 111A to move in a first direction and thegeometry of slots 116 and 126 translate axle ends 111A to generate askew condition in the variator. The geometry of slots 116 and 126 allowsends 111A to translate in slots 116 to target tilt angle 220 b.Eventually, the original rotation of carrier 114 or 124 and the angularmovement of axle ends 111A due to slots 116 and 126 will offset andplanet axles 111 will have zero skew angle at the target tilt angle.When the actual skew angle and the desired skew angle are equal andplanet axes of rotation 115 are parallel to axis of rotation 105, theplanet axle angle will stop changing (i.e. the system is stable). FIG.2A depicts this principle.

FIG. 2B depicts a diagram of skew angle and tilt angle .gamma. (gamma)over time, illustrating a relationship between skew angle and tilt angleduring reverse operation. During reverse rotation, if a first carrier(e.g., carrier 114) is rotated to achieve a skew angle (depicted aspoint 210 b), a skew condition of the planet axes of rotation 115 willcause planet axle ends 111A to rotate in a first direction, but slots116 may allow planet axle ends 111A to move further radially inward,indicated by line 221. Because slots 116 are configured with slot angle.THETA. (theta) for rotation in a design direction, as planet axle ends111A move radially inward, slots 116 will cause planet axle ends 111A tomove in a second direction. This motion caused by slots 116 will add tothe original rotation of first carrier 114, indicated by line 210 d. Theskew angle 210 d of planet axes of rotation 115 will increase and theimpetus for planet axle ends 111A in first carrier 114 to move radiallyinward will increase. FIG. 2B depicts a diagram illustrating thisprinciple. Eventually, the skew angle .zeta. (zeta) of planet axes ofrotation 115 from the rolling direction will increase to a maximum value222 to cause the tilt angle to reach a maximum value 223 such that thetransmission torque loss will overcome the available drive torque (i.e.,the system is unstable). Moreover, if the scenario is left unchecked,sliding action caused by a planet whose axis of rotation is radicallyskewed from the rolling direction may destroy the rolling contacts orotherwise cause damage to the CVT, which may cause the CVT to fail.

Alternatively, during reverse rotation, if first carrier 114 is rotatedin the opposite direction, skew angle 210 of planet axes of rotation 115will cause planet axle ends 111A in first carrier slots 116 to moveradially outward. Because slot angle .THETA. (theta) of slots 116 isconfigured for rotation in a design direction, as planet axle ends 111Amoves radially outward, slots 116 will cause planet axle ends 111A tomove. This motion caused by slots 116 will add to the original rotationof first carrier 114 (i.e., bias first carrier 114 in the samedirection). The skew angle 210 d of planet axes of rotation 115 willincrease and the impetus for planet axle ends 111A at first carrier 114to move radially outward will increase. Eventually, the skew angle.zeta. (zeta) of planet axes of rotation 115 will reach a value 222 tocause the tilt angle .gamma. (gamma) to reach a value 223 such that thetransmission torque loss will overcome the available drive torque.Moreover, any sliding action, caused by any planet 110 whose axis ofrotation 115 is radically skewed from the rolling direction, maycontact, damage or destroy a rolling contact or other component of theCVP.

Embodiments disclosed herein may overcome these, and other limitationsof the prior art. Embodiments allow reverse rotation in a ball typevariator utilizing skew control and angled slots in carriers 114, 124.Skew control and angled slots 116, 126 in carriers 114 and 124 providenegative feedback to planet axis angle change when in forward rotation.When operating in a reverse rotation, the rotation angle of firstcarrier 114 relative to second carrier 124 is actively controlled suchthat the skew angle of planet axes of rotation 115 relative to therolling direction is controlled. For example, consider that duringreverse rotation, tilt angle .gamma. (gamma) of planet axes of rotation115 is to be adjusted such that axle ends 111A at first carrier 114Amove radially inward by a small amount. FIGS. 3A and 3B depict diagramsof skew angle and tilt angle over time, illustrating how tilt angle.gamma. (gamma) may be changed even when a CVT is operating in a reversedirection.

Embodiments for controlling a tilt angle during reverse rotationdisclosed herein may include a processor communicatively coupled to anactuator and a memory storing a program or a set of instructionsexecutable by the processor. The processor may perform a method ofcontrolling or managing a CVP, a variator, a CVT subassembly, a CVT, adrivetrain or a vehicle having a CVT.

FIG. 3A depicts a flow diagram, illustrating one embodiment of a methodfor controlling rollback in a CVT. As depicted in FIG. 3A, if an initialrotation of first carrier 114 causes a first skew angle 310 a to cause afirst tilt angle rate of change 340 a to a first tilt angle 340 b, thenafter the initial rotation to a first skew angle 310 b, while axle ends111A move radially inward, first carrier 114 may be rotated in anopposite direction according to a second skew angle rate of change 310 cto a second skew angle 310 d to compensate for the effect that angledslots 116 have on the skew of planet axes of rotation 115, and skewangle rate of change 310 e is held constant for a desired time or untiltilt angle .gamma. (gamma) achieves a target value 340 d. Note thatalthough skew angle rate of change 310 e is constant, tilt angle 340 cmight change. In other words, a rotation angle for first carrier 114does not equal the target tilt angle. When the desired movement has beenmade or the tilt angle .gamma. (gamma) nears a target tilt angle 340 e,an additional rotation 310 g of first carrier 114 in the clockwisedirection is required to return planet axes of rotation 115 to zero skew310 h in the rolling direction.

As depicted in FIG. 3B, if an initial rotation 310 a of first carrier114 adjusts CVP at a first tilt angle rate of change 340 a to a firsttilt angle 340 b, then after the initial rotation to a first skew angle310 b, while axle ends 111A move radially inward, first carrier 114 maybe rotated in an opposite direction according to a second skew anglerate of change 310 c to a second skew angle 310 d to compensate for theeffect that angled slots 116 have on the skew of the planet axes ofrotation 115. Skew angle rate of change 310 may be adjusted using aseries 310 e-1 to 310 e-n or until tilt angle .gamma. (gamma) achievestarget value 340 d (or nears target value 340 d). When the desiredmovement has been made or when the CVT is operated in a designdirection, an additional rotation 310 g of first carrier 114 in thefirst direction may return planet axes 115 to zero skew 310 h in therolling direction. Tilt angle 340 remains at the target tilt angle untilanother set of commands.

A CVT that is started in a forward direction, started from stop, orstarted in a reverse direction may be controlled using an active controlalgorithm. The control logic for a skew control based planetary CVT withmechanical gamma feedback and which allows reverse rotation mightinclude determining a current transmission ratio, such as by a storedvalue from another operation or the previous measurement, determining acurrent skew such as from the last observed rotation change and rotationvalues, (speed and direction), or determining the current direction ofrotation and speed of rotation. If the rotation direction is reversed,or zero, or expected to be reverse, the relative carrier angle may berotated to a position such that the sum of the last known skew and therotation amount result in a skew value that would safely initiate adownward rotation in reverse rotation. As long as the CVT is operatingin a reverse direction, control may include determining an actual skewdirection and rate of change of the tilt angle gamma as well as rotationdirection, and correcting the skew for selected conditions.

FIG. 4 depicts a flow diagram, illustrating one method for controllingtilt angle of a CVT. As depicted in FIG. 4, the current value of planetaxis skew angle and direction of rotation are obtained, the load ismonitored, and as the ratio changes in the desired direction, subsequentsignals may be communicated to maintain, reduce or reverse the skewangle an appropriate amount to control the rate of ratio change.Furthermore, embodiments may be preset to operate in a reverse directionas a precaution.

In step 410, a processor may receive, sense, or otherwise obtaininformation about a current value of planet axis skew angle and adirection of rotation. Planet axis skew angle may be known bydetermining a rate of change of planet axis skew angle and a rate ofrotation of planets 110. A rate of change of the planet axis skew anglemay be determined from a rate of change of a transmission ratio or otherrelative factors. Since the creep of the rolling surfaces, andsubsequent loss of rolling speed, is related to the torque and speed ofthe transmission, calculation of the rate of change in planet axis angleis generally affected by the power. Hence, the power is one of therelative factors. Rotation direction can be determined by measurement ofthe phase angle between two offset signals such as inductive or HallEffect speed pickups. Rotation direction can also be indicated byobserving the direction of the actual change in transmission ratioversus an expected change. For example, if a signal is input to rotatethe first carrier relative the second carrier to increase the ratio, butit decreases instead, that may be an indication that the rotationdirection is the opposite of the expected rotation direction.

In some embodiments, step 415 includes monitoring the load on the CVT.Noting that the torque on the transmission causes load at each elementin the control path, then backlash and compliance in the controlelements, as well as changes in the creep rate, might affect theconclusion. Take for example the case where a signal to decrease ratiotowards overdrive is synchronized with an increase in externalload/torque on the transmission. A rotation of the relative angles ofthe carriers and subsequent desired change in skew angle of the planetaxis might be expected to rotate the transmission towards overdrive.However, the increase in applied load might cause enough deflection inthe elements to cause the actual skew angle to be opposite in sign. Theresult might be a negative rotation when a positive rotation wasexpected (or vice versa). Thus, the load on the transmission may bemonitored and considered if rotation direction is to be determined fromactual change in ratio versus expected change in ratio.

In step 420, a signal is sent to adjust carrier angle to provide adesired skew angle and therefore achieve a target tilt angle. Underforward operating conditions, steps 410, 415 and 420 are continuouslyperformed to provide continuous and stepless transmission speed ratios.

Under reverse operations, steps 410, 415, 420 and 425 are continuouslyperformed to provide continuous and stepless transmission speed ratios.In particular, any change of a planet axis 115 from a zero skew anglewhen the drive direction is opposite of design may likely cause arunaway move (i.e., an end 111A or 111B of one or more planet axles 111will tend to translate along slot 116 or 126 towards one of the ratioextremes because of positive ratio feedback). In step 425, a signal toreverse carrier rotation is sent to an actuator. If carriers 114 or 124are rotated such that reversed rotation is certain to move planet axes115 towards underdrive, then as planets 110 move towards underdrive, oneor both carriers 114, 124 may be rotated to an overdrive condition tocompensate for the positive reinforcement of the underdrive rotationcaused by slots 116 in carrier 114 or 124.

A CVT may be started from stop. Whenever the transmission is stopped, orthe next action is indeterminate, a command for offsetting skew in thedirection that will result in a rotation towards reduction if rotationdirection is reverse may be communicated to an actuator. In a preferredembodiment, a command for offsetting the skew in the direction that willresult in a rotation towards reduction if rotation direction is reversethat is adequate to overcome any inaccuracies in skew position due tomachining error or droop in skew position due to load is communicated toan actuator. If the CVT is started from stop and the CVT is set tooperate in a forward rotation direction but instead operates in areverse rotation direction, damage may occur. In some embodiments, instep 430, the CVT is preset to operate in a reverse rotation direction.Thereafter, if the CVT is operated in a reverse mode, a command may begiven to adjust the carrier angle (step 420) and embodiments mayimmediately begin monitoring the CVT to obtain information about thecurrent value of planet axis skew and direction of rotation (step 410)to provide feedback to maintain a stable system. Alternatively, if theCVT is operated in a forward direction, slots 116 or other geometry ofcarriers 114 and 124 immediately provide positive feedback to maintain astable system. Advantageously, the potential for damage to the system isreduced.

In some embodiments, a signal (e.g., a signal as sent in steps 420 or425 or information obtained by monitoring a load in step 415) may besent to an actuator to maintain the present skew angle of a CVT. Theactuator may maintain this skew angle until subsequent signals arecommunicated to the actuator. Rotating may be accomplished by aprocessor sending a signal to an actuator coupled to carrier 114 or 124.In some embodiments, an actuator may be coupled to both carriers 114 and124, and changing the skew angle may involve coordinating the rotationalposition of carriers 114 and 124.

The rate at which negative feedback is provided by an actuator may bemore than the rate at which feedback is provided by the slots in forwardoperation. For example, the feedback provided by the slots depends on,among other things, the widths of the slots. As such, wider slots mayprovide less feedback. In other embodiments, the amount of negativefeedback may be based on a parameter of the slots along with a speed ofthe CVP, a speed ratio (SR) of the CVP, a tilt angle of the CVP, or someother parameter determined to have an effect on the likelihood of theCVP adjusting to an undesired operating condition. Thus, if a CVP isoperating at a high speed and slots 116, 126 have greater tolerances,more feedback (including higher frequency or greater magnitude) may berequired to prevent damage, but a CVP operating at low speeds or withtighter tolerances may require less feedback.

As disclosed above, a system utilizing an active control algorithm maybe useful for stabilizing a CVT in either rotation direction. Inaddition to controlling rollback by using continuous adjustments afteran initial rotation, embodiments may include systems for controllingconditions which could lead to rollback. In some embodiments, a thirdplate with a third set of slots may be used as a timing plate. A timingplate may partially synchronize the ratio angles and the skew angles ofeach planet 110 within the plurality of planets 110.

FIGS. 5A, 5B and 5C depict views of carriers 114 and timing plate 510.It should be noted that carrier 124 is not shown, but that carrier 114and carrier 124 are similar and may be identical. Accordingly, onlycarrier 114 is described here for simplicity and ease of understanding.Also, it should be noted that carrier 114 and timing plate 510 aredepicted in FIGS. 5A and 5B as mirror images. However, this is just forease of description and the dimensions of timing plate 510 may differ.For example, a thickness of timing plate 510 may be less than athickness for carrier 114, the value of an angle for slot 516 may bemore than, less than, or the same as the value of an angle for slot 116,the width (W.sub.516) of slot 516 may be greater than, the same, or lessthan the width (W.sub.116) of slot 116, the length (L.sub.516) of slot516 may be longer or shorter than the length (L.sub.116) of slot 116,etc. In some embodiments, width W.sub.516 of slot 516 has tolerancesselected such that slots 116 or 126 are the primary structures forcontrolling planet axle ends 111A or 111B in forward and reverseoperations and slots 516 are for preventing runaway or other effects inreverse operation. In some embodiments, the value of an angle, thewidth, or some other parameter of slot 516 is selected such that slots116 or 126 are the primary structures for controlling planet axle ends111A or 111B in forward operations, and slots 516 are intended forpreventing runaway or other effects in reverse operation.

FIGS. 5C and 5D depict views of one embodiment of a system includingcarrier plates 114A and 114B with control via a floating timing plate510 interposed between carrier plates 114A and 114B. As depicted inFIGS. 5C and 5D, planet axles 111 extend through timing plate slots 516and engaging carrier slots (e.g., carrier slots 116A and 116B). If anangle (B) between timing plate slot 516 and corresponding carrier slot116 is other than zero, then one end (111A or 111B) of planet axle 111may be positioned at the intersection (L) of timing plate slot 516 andcarrier slot 116. In this configuration, timing plate 510 may be usefulfor controlling the movement of planet axles 111 along carrier slots116, such as preventing any single planet axle 111 from deviating in adirection from the other planet axles 111, or maintaining each planetaxle 111 at an angle within a tolerance of a collective angle for theplurality of planet axles 111. For example, if the collective angle is25 degrees, embodiments may ensure all axles are at an angle between 22degrees and 28 degrees. It is advantageous for efficient forwardoperation that slots 516 in each of carriers 114, 124 synchronize theangular spacing among the ends of each planets planet axle 111 withinthe array of planets 110. Embodiments disclosed herein provide anadequate amount of clearance or backlash in the timing plate slots toprevent timing plate 510 from interfering with the operation of eitherof the carriers (114 or 124).

The driving angle (alpha) refers to the angle between the projection ofeach of the timing plate slots 516 and a line 515 radial to the centeraxis and intersecting the centers of the timing plate slots 516 at thepitch diameter of the array of planets wherein the projection plane isnormal to center axis 105. Blocking angle (B), as used herein, may referto the angle between the projection of the timing plate slot centerlinesand the carrier slot centers at the intersection of the timing plateslot centerlines, the carrier guide slot centerlines and the pitchdiameter of the array of planets wherein the projection plane is normalto the center axis. The optimal blocking angle occurs when timing plateslots 516 are 90.degree. to carrier slots 116 and 126 (opposite thedirection of rotation).

Timing plate 510 may be free running or may be grounded relative to acarrier (e.g., carrier 114 or 124). In embodiments in which timing plate510 is free running, its angular position may be determined by the sumof forces of the array of planet axles 111. In a preferred embodiment,the driving angle for slots 516 in timing plate 510 ideally will be lessthan 90.degree. from radial. Advantageously, timing plate 510 mayprevent a large error in ratio or skew by blocking the change of ratioangle. In a preferred embodiment, from the dual considerations forblocking and driving, timing plate slots 516 are configured such thatthe blocking angle is no less than 30.degree. (relative to a radial lineand in the design direction of rotation) and the driving angle nogreater than 45.degree. (relative to a radial line and opposite thedesign direction of rotation).

In some embodiments, timing plate 510 having radial slots 516 mayenhance the control by ensuring each planet 110 in the array of planets110 is within a controlled ratio angle of the set of planets and withina limited skew angle of the set of planets 110. Timing plate 510 may bea free turning disc with radial slots 516. In some embodiments, timingplate 510 may be positioned axially between the carrier halves 114 and124. Each of the planet axles 111 passes through timing plate 510 beforeengaging the carrier guide slots 116 or 126 at one end of the drive.Tolerances of timing plate slots 516 allow carrier slots 116 or 126 tobe the primary control of axles 111 and the primary circumferentialalignment feature for planets 110. In some embodiments, the tolerancesallow deviations of up to 3 degrees. In other embodiments, thetolerances allow for deviations up to 5 degrees.

In some embodiments, timing plate 510 having offset radial slots 516 mayenhance the control by ensuring each planet axle 111 for all planetaxles 111 in the array of planets 110 is within a controlled ratio angleof a mean ratio angle of the plurality of planets and within a limitedskew angle of a mean ratio angle of the plurality of planets.

In some embodiments, timing plate 510 may comprise a disc with offsetradial slots 516 formed therein, and may be positioned axially outsideof one of carriers 114 or 124 and driven by carrier 114 or 124 oppositeits axial position relative to planets 110. Each of the planet axles 111extends through a carrier guide slot 116 or 126 and engages a timingplate slot 516. In this configuration, timing plate slots 516 has largertolerances. However, carrier guide slots 116 or 126 have sufficienttolerances for planet axles 111 to allow timing plate slots 516 to bethe primary circumferential alignment feature for planets 110. The angleof timing plate slots 516 may be determined as a function of the anglesof carrier guide slots 116 and 126 in both carriers 114 and 124.

There are considerations which may affect the choice of the offsetradial slot angles for a free running timing plate 510. Theseconsiderations include, but are not limited to, minimizing the timingplate drive torque, maximizing the synchronizing force, and minimizingthe backlash or allowed synchronization error. Some factors which mayaffect these considerations include: manufacturing variations andtolerance bands for carrier guide slot radial spacing; carrier guideslot width; timing plate guide slot radial spacing; timing plate guideslot width; and axle or axle endcap diameters. Control factors such as adesired stationary skew value or a minimum desired continuous skew inreverse operation may also be of interest in timing plate offset radialangle design.

In some configurations, it may be necessary or desirable to have atiming plate driven by a carrier positioned proximate to the timingplate (i.e., located axially on the same side of a plurality ofplanets). In these configurations, the timing plate and the drivingplate may be coupled via a mechanism such that an angular movement ofthe driving plate in a first direction is counteracted by an angularmovement of the timing plate in the opposite direction. In someembodiments, a timing plate may have a first gear with a first set ofteeth, and a carrier may have a second gear with a second set of teethfor meshing with the first set of teeth. As the carrier rotates, thesecond gear rotates in a first direction while the second set of teethare engaged with the first set of teeth on the first gear, which causesthe first gear to rotate in an opposite direction to bias the timingplate. Other mechanisms may be possible.

A method of manufacturing a timing plate for controlling rollback in aCVT capable of reverse operation may include forming a plurality oftiming plate slots (e.g., slots 516) in a timing plate, wherein theplurality of timing plate slots are formed at an angle relative to aplurality of carrier guide slots (e.g., slots 116 or 126). The angle maybe determined based on an analysis for optimizing a synchronizationforce (i.e., the force necessary to prevent one or more runaway planetsfrom affecting the array of planets). In some embodiments, forming theplurality of timing plate slots includes determining an angle (b)between the timing plate slots and a radial line, wherein both intersectat the planet array pitch radius at an angle of 0.degree. (i.e.,perpendicular to a planet axis). The angle may be determined based on ananalysis of criteria to minimize a skew or tilt force (i.e., a forcenecessary to effect a desired skew angle or tilt angle). In someembodiments, the method may further include determining an angle betweenthe timing plate slots and a radial line of between 30.degree. and60.degree., where both intersect at the planet array pitch radius. Theangle may be determined based on a compromise between any of thepreceding factors. Furthermore, in some embodiments, the angle betweenthe timing plate slots and a radial line where both intersect at theplanet array pitch radius of between 0.degree. and 80.degree. isprotected for the maximum possible contributions of all the previouslydescribed factors.

In some embodiments, a targeted continuous operating condition forrotation opposite design comprises a position such that the planet axlesor planet axle endcaps nearest the input end of the CVP contact thecentermost limit of the input carrier's guide slot. In some embodiments,a skew angle may be limited to a minimum angle required to maintain areduction rotation effort for each of the planets in the array ofplanets. A minimum skew angle may be determined based on machiningtolerances (including errors or other variations) and may furtheraccount for changes in external load or ratio droop.

In some embodiments, all but one of the array of planets may be heldwith a small amount of positive skew, which may allow the system togradually change ratio in an overdrive direction. The remaining planetmay be held at a position with a small amount of negative skew. However,the planet with negative skew is prevented from having additionalnegative skew or from negating the positive skew of the remainingplanets.

Embodiments disclosed herein have been described as they pertain toplanetary type continuously variable transmissions. Furthermore,embodiments have been depicted with power entering through a shaft.However, those skilled in the art will appreciate that concepts andfeatures described herein may be applicable to other settings, includingpower entering through a ring or some combination of rings and a shaft.Furthermore, embodiments disclosed herein may be used individually or incombination with other embodiments to provide a drive train,continuously variable transmission, variator or the like capable ofoperating in either a forward direction or a reverse direction. Thoseskilled in the art will appreciate that these concepts may be equallyuseful in other settings and are thus not to be limited.

What is claimed:
 1. A method for controlling tilt angle in a ballplanetary continuously variable transmission (CVT) comprising a firstcarrier comprising a plurality of first slots and a second carriercomprising a plurality of second slots and at least one planet axleextending from a first particular slot of the plurality of first slotsto a second particular slot of the plurality of second slots, the methodcomprising: operating the CVT in a design direction of rotation,including rotating the first carrier of the CVT in a first direction toa first skew angle of the planet axle associated with a desired tiltangle of the planet axle; and operating the CVT in a reverse directionof rotation, including rotating the first carrier of the CVT in a seconddirection opposite the first direction to a second skew angle of theplanet axle; monitoring the CVT to determine a change in a tilt angle ofthe planet axle; and rotating the first carrier in the first directionto a third skew angle of the planet axle, the third skew angle resultingin the CVT having the desired tilt angle of the planet axle.
 2. Themethod of claim 1, further comprising determining when the CVT hasswitched from the design direction to the reverse direction.
 3. Themethod of claim 1, wherein operating the CVT in the reverse rotationfurther comprises causing a series of additional changes to the skewangle of the planet axle in the first direction.
 4. The method of claim3, further comprising: determining, based on one or more of thedimensions of the plurality of first slots in the first carrier of theCVT and the dimensions of the plurality of second slots in the secondcarrier of the CVT, at least one additional change of the series ofadditional changes; and causing the at least one additional change. 5.The method of claim 1, wherein the CVT further comprises an array ofplanets orbital about a center axis of the CVT, each planet having anaxis of rotation, and wherein the method further comprises determiningthe skew angle based on the rate of change of a planet axis skew angleand the rate of rotation of the planets.
 6. The method of claim 1,further comprising determining a load change, wherein the skew angle ofthe planet axle is further changed to offset a bias caused by the loadchange, and wherein the bias causes the CVT to adjust a transmissionratio to either an overdrive condition or an underdrive condition.
 7. Acontinuously variable transmission (CVT), comprising: a variator,comprising an array of planets orbital about a longitudinal axis, eachplanet having a planet axle defining a planet axis of rotation, a firstring in contact with the array of planets and orbital about thelongitudinal axis, the first ring being on a first side of the array ofplanets, a second ring in contact with the array of planets and orbitalabout the longitudinal axis, the second ring being on a second side ofthe array of planets, a sun located radially inward of and in contactwith the array of planets, a first carrier comprising a plurality offirst slots, each first slot configured for receiving a first end of aplanet axle, and a second carrier opposite the first carrier andcomprising a plurality of second slots, each second slot configured forreceiving a second end of the planet axle; an actuator coupled to atleast one of the first carrier and the second carrier; and a controllercommunicatively coupled to the actuator, the controller comprising aprocessor and memory storing a set of instructions executable by theprocessor to perform determining if the variator is operating in adesign direction or a reverse direction; the controller being configuredto, when the variator is operating in a design direction, determine adesired tilt angle for the array of planets and send a signal to theactuator to rotate the first carrier in a first direction to a firstskew angle; and the controller being configured to, when the variator isoperating in a reverse direction, determine the desired tilt angle forthe array of planets, send a first signal to the actuator to rotate thefirst carrier in a second direction opposite the first direction to asecond skew angle, monitor the CVT to determine a change in the tiltangle, and send a second signal to rotate the first carrier in the firstdirection to a third skew angle that adjusts the CVT to the desired tiltangle.
 8. The CVT of claim 7, wherein the set of instructions executableby the processor includes instructions for sending a series ofadditional signals to change the skew angle in the first direction. 9.The CVT of claim 7, wherein the set of instructions executable by theprocessor includes instructions for periodically changing the skew angleto offset a bias of the planets to tilt towards reduction.
 10. The CVTof claim 7, wherein the controller is configured to send an instructionto advance the skew angle.
 11. The CVT of claim 7, wherein when the CVTis stopped, the controller is configured to preset the variator tooperate in the reverse direction.
 12. The CVT of claim 7, wherein thecontroller is configured to change the skew angle based on one or moreof the dimensions of the plurality of first slots of the first carrierand the dimensions of the plurality of second slots of the secondcarrier.
 13. The CVT of claim 7, wherein a change in the skew angleafter a first rotation is based on a rate of change of planet axis skewangle and a rate of rotation of the planets.
 14. The CVT of claim 7,wherein the set of instructions executable by the processor includesinstructions for determining a load change, and wherein the skew angleis further changed to offset bias of the planets to tilt towardsreduction caused by the load change.